@knitsybitsy
Your sim has to be like lvl 7 or something in gardening to be able to tend all the plants, is your sim that? Otherwise I don't know, maybe the gardening is slower on community lots because "the sims are unfamiliar with the plants". This would be how I could explain it to myself lol.
Maybe you could have a big lot that is still residential with a mini-house and a sim that lives there that is "Park Caretaker" (this could be the sims name!), You would have to travel to the lot to "visit them", but I guess you could as well just hang out in "their" garden :P